The "boezem" in Dutch water management

The boezemstelsel is a contiguous array of aquatic bodies that holds excess water until release above sea-level into a river or the sea, and occasionally serves as reservoir for maintenance of the water table at polder-bottom in a dry season.

A polder is the unit of land enclosed by dikes. Its water table is explicitly prescribed, and meticulously regulated. A boezem is the variable body of water outside of and above this perimeter that buffers that regulation. Boezemstelsel means boezem system, and each is a separate hydrological entity.

Simple in concept, a boezem often consists of a wide network of diverse bodies of water — canals, lakes, shipping channels, ditches — a network shared by several polders, and even a small region. It's a closed system with a common water level — with slight variations due to flow direction.

Ultimately, all water in the Netherlands must flow to the sea or evaporate. This includes the rainfall runoff of a large part of northwestern Europe.

Electric pumps — formerly windmills, steam and then diesel engines — elevate water from the polder floor to the boezem. The boezem holds the water until agreed measurements allow its release — usually into a river, and usually upward. Volume, speed, and quality of the output must be considered.

The water in the boezem can also be allowed back into the polder in the management of ground water there. The rich organic nature of much of western Netherlands' soil means that if it's exposed to oxygen, it will decompose — causing the land to subside. (And it has — this is the reason that delta land is now below sea-level.)

And, of course, agriculture always needs water. In a drought, the boezem can stabilize the supply.

Mostly, though, a boezem holds water pumped out of the polder and releases it (by sluice or further pumping) into the wider ecosystem on its way to sea.

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